"prerender: false" is ignored in child components

[halter73]

prerender: false in new InteractiveWebAssemblyRenderMode(prerender: false) and its Server/Auto counterparts only seems to have an effect for top-level render modes. If a parent component specifies a render mode, the prerender configuration of all its children are completely ignored.

#53462 reported a similar issue where prerender: true is ignored for child components if a parent component specifies prerender: false. This was closed as by design. IIt makes sense for that particular scenario, because if a child component can pre-render, that doesn't necessarily mean it needs to. And there's no way for the Blazor to know what child components exist during prerendering if the parent is not prerendered, so it's not even detectable until later during interactive rendering.

Ignoring prerender: false is worse mostly because it is possible to not prerender these child components during its parent's prerendering pass, so Blazor developers could easily expect this to work. Furthermore, it's likely that a child component that explicitly asks for prerender: false is doing something like JS interop which fails during prerendering.

If we really cannot make this work, it should at least raise an error like we do when a child component tries to specify a rendermode that incompatible with its parent. (e.g. System.NotSupportedException: Cannot create a component of type 'BlazorApp1.Client.Pages.Counter' because its render mode 'Microsoft.AspNetCore.Components.Web.InteractiveWebAssemblyRenderMode' is not supported by interactive server-side rendering.) I'd prefer to make this scenario work though.

halter73/BlazorChildComponentPrerenderFalse@bb6108e shows the modifications you can make to the dotnet new blazor -int WebAssembly project template to demonstrate the error. It updates the Counter component to include prerender: false and adds if (!OperatingSystem.IsBrowser()) throw ... in OnInitialize. This will not throw if you navigate directly to /counter, but it also adds Counter as a child to CounterParent which allows prerendering. So, if you navigate to /counter-parent, you'll see the exception.

This repro just happens to be based on the WebAssembly project template, but the same problem exists for interactive server and auto rendermodes.

[guardrex]

UPDATE (6/18): I'm adding some remarks to the Render Modes article to cover this.

[lofcz]

Is the "parent component specifies prerender: true, child component prerender: false, the child gets pre-rendered anyway" part of this something the Blazor team think they can solve?

We ran into this with Server Interactive app, where we need the landing page prerendered and the post-authorization pages not. Since this was scheduled for .NET 10 and many core issues were pushed back from 9 to 10, I'm asking whether this is something that could be solved or if there is no point in waiting for the feature.

[nw5428]

I encountered this same issue where I have certain pages where I have configured for pre rendering and other pages where I have not. I investigated how [ExcludeFromInteractiveRouting] works and mimicked to accomplish what I was wanting. I describe what I did below, which only work for the main page you are navigating to. If you have a child component of a main navigation page then it will not work. Other then that it works great. I think this belongs in the framework itself.

I added to my client project
public class ExcludeFromPrerender : Attribute { }

Then the below to my Server project:

public static class PreRenderExtension
    {
        public static bool AcceptsPrerender(this HttpContext context)
        {
            ArgumentNullException.ThrowIfNull(context);

            var pageType = context.GetEndpoint()?.Metadata.GetMetadata<ComponentTypeMetadata>()?.Type;

            return pageType is not null
                && pageType.IsDefined(typeof(ExcludeFromPrerender), false);
        }
    }

Then added the below in App.Razor:

@code {
    private IComponentRenderMode _defaultRenderMode = InteractiveWebAssembly;

    private IComponentRenderMode? PageRenderMode
    {
        get
        {
            if(HttpContext.AcceptsPrerender() && _defaultRenderMode == InteractiveWebAssembly)
            {
                return new InteractiveWebAssemblyRenderMode(prerender: false);
            }
            else
            {
                return _defaultRenderMode;
            }
        }
    }
}

My App.razor then has:

<Routes @rendermode="PageRenderMode" />
<HeadOutlet @rendermode="PageRenderMode" />

Then I finally add: @Attribute [ExcludeFromPrerender] to my component.